Cruises can go on sale as much as two years in advance (hello, spring break 2018!), and there are great deals to be had if you plan ahead and book far out. But if you are a procrastinator — or really need to get away RIGHT NOW — jaw-droppingly low fares can be found just weeks before the ship sets sail. So what's the catch? First, know that cruise lines consider a ship full if there are at least two people in every stateroom. About 90 days before a sailing, a cruise line will start to panic if there are lots of empty cabins and begin slashing prices, and keep lowering the fare as the sailing date gets closer. That's when you start to see fares from $449 per person or less for a one-week Caribbean sailing. That's a hard price to pass up, but you need to know the caveats. The good news is, most things on a cruise ships are open to everyone. Which means you'll get basically the same vacation as everyone else on board who may have booked at a much higher price. Here's what you need to know before you jump on a last-minute deal.